MIC74 datasheet(14/20 Pages) MICREL

14 / 20 page

MIC74

Micrel

MIC74

August 1, 2000

Read

STATUS

x set

Yes

Polled

I.S.R.

Service function

STATUS

≠ 00

Yes

m set

Yes

Service function

n set

Yes

Service function

Service other devices

Return from ISR

Figure 12: Interrupt Service Routine Without ARA

reading STATUS clears it in preparation for detecting future

events. The status bits corresponding to I/O’s configured as

outputs or corresponding to P[7:4] when in fan mode will not

be set by state changes on these pins. It is always good

practice, however, for the interrupt service routine to mask

the value obtained when reading STATUS to eliminate any

bits, output or otherwise, that are not of immediate concern.

This will help avoid problems if software changes are made

in the future.

The process for writing output data is straightforward—

simply write the desired bit pattern to DATA. Special precau-

tions may be required, however, when changing output data

in an interrupt driven system. See the discussion below under

“Writing to the Data Register.”

Read STATUS to

determine source

Read alert

response address

Interrupts

pending

Yes

x set

Yes

Interrupt Service

Routine

Return from ISR

Service function

interrupt

from

MIC74

Yes

Service other devices

m set

Yes

Service function

n set

Yes

Service function

Figure 11: Interrupt Service Routine Using the ARA

Writing To The Data Register

Multiple software routines may use the various output bits

available on the MIC74 to control individual functions such as

power switches, LED’s, etc. These various functions may be

handled by independent software routines which must ma-

nipulate individual output bits without regard for other bits.

Care must be taken to insure that these various software

routines do not interfere with each other when modifying

output data. The recommended procedure for changing

isolated output bits is as follows:

1. Read DATA

2. Set desired bits by ORing the value read from

DATA with an appropriate mask value

3. Clear desired bits by ANDing the value read

from DATA with an appropriate mask value

4. Write the result back to DATA

A functionally equivalent alternative to this procedure is to

keep an image of the data register in software. Any indepen-

dent routines would make changes to this image using the

procedure above and then call a routine that actually writes

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